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Thermal Alteration of CM Carbonaceous Chondrites: Mineralogical Changes and Metamorphic Temperatures

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Thermal Alteration of CM Carbonaceous Chondrites: Mineralogical Changes and Metamorphic Temperatures Thermal alteration of CM carbonaceous chondrites: mineralogical changes and metamorphic temperatures A. J. King1,2*, P. F. Schofield1 and S. S. Russell1 1Planetary Materials Group, Department of Earth Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD, UK. 2School of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK. *corresponding author: A. J. King ([email protected]) The CM carbonaceous chondrite meteorites provide a record of low temperature (<150°C) aqueous reactions in the early solar system. A number of CM chondrites also experienced short-lived, post-hydration thermal metamorphism at temperatures of ~200°C to >750°C. The exact conditions of thermal metamorphism and the relationship between the unheated and heated CM chondrites are not well constrained but are crucial to understanding the formation and evolution of hydrous asteroids. Here we have used position-sensitive-detector X-ray diffraction (PSD-XRD), thermogravimetric analysis (TGA) and transmission infrared (IR) spectroscopy to characterise the mineralogy and water contents of 14 heated CM and ungrouped carbonaceous chondrites. We show that heated CM chondrites underwent the same degree of aqueous alteration as the unheated CMs, however upon thermal metamorphism their mineralogy initially (300 ‒ 500°C) changed from hydrated phyllosilicates to a dehydrated amorphous phyllosilicate phase. At higher temperatures (>500°C) we observe recrystallisation of olivine and Fe-sulphides and the formation of metal. Thermal metamorphism also caused the water contents of heated CM chondrites to decrease from ~13 wt% to ~3 wt% and a subsequent reduction in the intensity of the 3 µm feature in IR spectra. We estimate that the heated CM chondrites have lost ~15 ‒ >65 % of the water they contained at the end of aqueous alteration. If impacts were the main cause of metamorphism, this is consistent with shock pressures of ~20 ‒ 50 GPa. However, not all heated CM chondrites retain shock features 1 suggesting that some were instead heated by solar radiation. Evidence from the Hayabusa2 and ORSIRS-REx missions suggest that dehydrated materials may be common on the surfaces of primitive asteroids and our results will support upcoming analysis of samples returned from asteroids Ryugu and Bennu. 1. INTRODUCTION The CM (“Mighei-type”) carbonaceous chondrite meteorites are chemically amongst the most primitive extraterrestrial materials available for study (e.g. Brearley 2006). However, the CM parent body originated beyond the water snowline (~1‒3 AU) and contained ices that melted to produce fluids (Miyamoto 1991; Palguta et al. 2010; Bland and Travis 2017). This led to varying degrees of aqueous alteration that transformed anhydrous silicates and metal into a secondary mineral assemblage of abundant (>60 vol%) phyllosilicates and minor amounts (<5 vol%) of oxides, sulphides, and carbonates (Zolensky et al. 1997; Rubin et al. 2007; Howard et al. 2009, 2011). Evidence from the Mn-Cr ages of carbonate grains suggest that the CM parent body accreted to >60 km in diameter ~3.5 million years after the formation of calcium-aluminium-rich inclusions (CAIs) (Fujiya et al. 2012). The mineralogical and chemical characteristics of CM chondrites indicate that the aqueous alteration proceeded at low temperatures (<150°C) (Clayton and Mayeda 1984; Baker et al. 2002; Benedix et al. 2003; Guo and Eiler 2007; Verdier-Paoletti et al. 2017; Fujiya 2018; Vacher et al. 2019) during the first ~10 million years of the solar system (Endress et al. 1996; de Leuw et al. 2009; Fujiya et al. 2012). As samples of a hydrous asteroid, the CM chondrites are an important tool in efforts to understand the nature, distribution, and transport of water in the protoplanetary disk. In the global meteorite collection, there are a number of CM chondrites that following aqueous alteration also experienced thermal metamorphism. First reported in the 1980s (e.g. Skirius et al. 1986; Akai 1988; Tomeoka et al. 1989), heated CM chondrites can be identified 2 from their unusual mineralogy, elemental and isotopic compositions, and organic chemistry relative to the unheated CM meteorites. For example, heated CM chondrites can contain an amorphous phase produced by the dehydration phyllosilicates, fine-grained, recrystallized olivine, and melted Fe-sulphide masses (Nakamura 2005; Tonui et al. 2014; Lee et al. 2016; King et al. 2019a). They are often depleted in water (Alexander et al. 2013; Garenne et al. 2014), light noble gases (e.g. 4He and 20Ne) (Nakamura 2006), and volatile trace elements (e.g. Cd) (Paul and Lipschutz 1990; Xiao and Lipschutz 1992; Wang and Lipschutz 1998; Lipschutz et al. 1999), while oxygen isotopic compositions can be shifted (Clayton and Mayeda 1999; Lindgren et al. 2020) and organics modified or destroyed (Kitajima et al. 2002; Quirico et al. 2018; Chan et al. 2019). By studying these different properties, and through comparison to the products of artificial heating experiments, it has been shown that some CM chondrites suffered relatively mild thermal metamorphism at temperatures of <500°C, whereas others were fully dehydrated and recrystallised at >750°C (Nakamura 2005; Tonui et al. 2014). The mechanism, timing and duration of the metamorphic event(s) that produced heated CM chondrites are not well constrained. The metamorphism must have happened after the dominant period of hydration had ceased because phyllosilicates are usually dehydrated, although it is not clear whether the process was a single event or episodic, and tentative evidence for retrograde aqueous alteration has been reported in at least two mildly heated CM chondrites (Quirico et al. 2018; Lee et al. 2019a). It is possible that at the end of aqueous alteration, temperatures in some regions of the parent body continued to rise, leading to metamorphism of the hydrated materials. In this scenario, the heat source would have been the radiogenic decay of 26Al (half-life of 0.7 million years), and metamorphism lasted for millions of years (Miyamoto 1991; Palguta et al. 2010; Fujiya et al. 2012). However, this timescale is inconsistent with estimates from the Fe-Mg diffusion between chondrules and matrix and the structure of organics in heated CM chondrites that suggest the duration of metamorphism was 3 much shorter, on the order of hours to several years (Nakato et al. 2008; Yabuta et al. 2010; Quirico et al. 2018). Hypervelocity impacts were a major geological process in the early solar system and could have generated the high temperatures on short timescales required to form heated CM chondrites. Collisions into the parent body would have initially compressed and heated the target rocks, with phyllosilicates continuing to be dehydrated by residual heat after the release of impact pressure (Tyburczy et al. 1986; Tomeoka et al. 1999; Rubin 2012; Bland et al. 2014; Lindgren et al. 2015; Lunning et al. 2016). Artificial shock experiments using both terrestrial minerals and meteorites indicate that impact pressures of 15 – 30 GPa are required to initiate dehydration of phyllosilicates (Tyburczy et al. 1986; Tomeoka et al. 1999). High impact pressures would have led to the catastrophic disruption of hydrous asteroids and the formation of distinctive shock features in the target rocks (Scott et al. 1992; Rubin 2012; Lindgren et al. 2015; Michel et al. 2015; Lunning et al. 2016; Jutzi et al. 2019), however clear evidence for shock is not always present in the heated CM chondrites (e.g. Tonui et al. 2014; Lee et al. 2016). An alternative additional source of heat for near-Earth asteroids (NEAs) is solar radiation, as has been proposed for Phaethon (Ohtsuka et al. 2006; Takir et al. 2020). The surface temperature of NEAs can reach >1000°C leaving hydrous asteroids with relatively homogenous dehydrated crusts (Chaumard et al. 2012; Nakamura et al. 2015; Kitazato et al. 2021). The visible and near-infrared (IR) reflectance spectra of heated CM chondrites closely resemble some low albedo C-type asteroids (Hiroi et al. 1993, 1996; Cloutis et al. 2012), although it has been suggested that rather than being dehydrated these bodies never experienced widespread aqueous alteration (Vernazza et al. 2015; Rivkin et al. 2019). Nevertheless, estimates of the surface water content of many C-type asteroids fall within the range of heated CM chondrites (Rivkin et al. 2003; Beck et al. 2021). These relationships suggest that materials 4 mineralogically and compositionally similar to heated CM chondrites could be a major constituent of the regolith on primitive asteroids. This argument is further strengthened by the initial results of JAXA’s Hayabusa2 and NASA’s OSIRIS-REx missions, which indicate that the surfaces of the near-Earth carbonaceous asteroids Ryugu and Bennu contain mixtures of anhydrous, hydrous and dehydrated rocks (Kitazato et al. 2019, 2021; Hamilton et al. 2019; DellaGiustina et al. 2021). Quantifying the extent of aqueous and thermal alteration in the heated CM chondrites is therefore a critical step for interpreting the geological history of the samples returned from Ryugu and Bennu. In this study we have investigated 13 CM and one C2ung carbonaceous chondrites reported in the literature as having experienced aqueous alteration followed by short-lived thermal metamorphism. We have characterised each meteorite using position-sensitive- detector X-ray diffraction (PSD-XRD), thermogravimetric analysis (TGA) and transmission IR spectroscopy. XRD is a sensitive indicator of the presence of dehydrated amorphous
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